Silver nitrate has many important industrial applications, for example, it is required for the manufacture of photographic materials, for certain catalytic uses, and for use in the pharmaceutical industry. In the process that is generally used for preparing silver nitrate, an aqueous silver nitrate solution is prepared and introduced to a cooling crystallizer. Cooling of the solution induces silver nitrate crystal growth. Solution containing silver nitrate crystals is then drawn from the crystallizer and the silver nitrate crystals are separated and dried.
Prior art processes suffer from serious deficiencies that have hindered their industrial utility. For example, a growth crystallizer such as a continuous cooling crystallizer typically has a residence time of about 12 to 24 hours, because low supersaturation is required to avoid fouling of the heat transfer surfaces. At shorter residence times and higher supersaturation, product begins to build up on cool metal surfaces, leading to fouling and blockage of the heat exchanger. A commercial size silver nitrate cooling crystallizer may therefore have to be shut down frequently, e.g. every 24 hours, for steam cleaning and defouling. Also, the process is time consuming because of the high residence time. This means that for a desired output of product, a large inventory of solution and therefore of silver must be maintained in the crystallizer. The size and the cost of the apparatus and facilities therefore is also high.
Silver nitrate crystals also have a tendency to fuse together especially if produced by evaporative crystallization, causing problems in its handling and its use. For example, fusing of a packaged crystal product creates a heavy, unwieldy block of material that can be hazardous to handle and prove difficult to redissolve for its intended use, such as in preparing photographic emulsions. The problem of fusing is accentuated as the mean particle size of the silver nitrate crystals decreases, thus exacerbating the problem for product produced by a process such as evaporative crystallization.
The present invention provides an industrially feasible and economically practical process that produces flowable silver nitrate while eliminating the requirement of a cooling crystallizer.